Mule Integration Workshop Mule Expression Language

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2

Chapters Schedule

Mule Expression Language(MEL) - Introduction

MEL - Syntax

MEL – Operand, Property

MEL – Arithmetic Operators

MEL – Comparison Operators

MEL – Logical Operators

MEL – Assignment, Literals

MEL – Literals

MEL – Maps, Lists and Arrays

MEL – Control Flow, Context Objects

MEL – Context Object – Server

MEL – Context Object – Mule, App

MEL – Context Object – Message

MEL – Variables

MEL – Data Extraction Function

Mule Expression Language(MEL) - Introduction Mule Expression Language (MEL) supports the work of message processors by

providing a means of accessing, manipulating, and using information from the message and its environment.

In most cases, Mule expressions work within message processors to modify the way those processors do their main jobs (for example, routing, filtering). Following are the principal use cases: Make a decision based on the contents, properties, or context of a message and its

attachments. For example, a flow controller can route purchase orders for different types of products to different JMS queues.

Select a value from the contents, properties, or context of a message and its attachments. For example, a cloud connector might extract a specific piece of information from the current message to use as an argument.

Replace a token with an actual value. For example, a logger can extract information from the payload contents and place it into a logging message.

We can also use MEL to implement a program (script) for which we would otherwise use a programming language like Ruby, JavaScript, or Groovy. Scripts in those languages cannot access the message and its environment as conveniently as MEL can. In this case, MEL is the programming language for a custom message processor.

Mule Expression Language - Syntax

Mule Expression Language Syntax MEL expression combines one or more operands with zero or more operators in a Java-like

syntax and returns the resulting value. At the heart of MEL are property expressions of the form contextObject.property. These

provide easy access to properties of the Mule message and its environment. For example, the expression “message.payload” represents the payload property of the message context object. Its value is the message payload.

Java method invocations and assignments are the other common MEL expressions. In most cases, a MEL expression stands alone as the value of a configuration property of a

message processor. Mule evaluates the expression at runtime, and the message processor uses the result.

The syntax of a MEL program largely follows Java, but MEL implements dynamic typing by performing type coercion at runtime. Semicolons follow each statement except the last, whether the statements are on one line or separate lines. Return statements are usually unnecessary, because a MEL program returns the value of the last MEL statement executed.

MEL operators and basic operands are conventional and predictable (for example, 2 + 2 == 4 returns true). Property expressions provide convenient access to information from the message and its environment (for example, server.fileSeparator returns "/" if the application is running on a Linux server, and "\" on a Windows server.). The remainder of this section summarizes the most important elements of MEL syntax.

Mule Expression Language – Operand, PropertyMEL Operand An operand can be a literal, a variable, or a MEL expression. The MEL expressions that most commonly appear as operands are property expressions and

method invocations.Property Expressions The syntax of a property expression is “contextObject.property”. This can appear as an operand

in most MEL expressions, including on the left side of an assignment if the property is writable from MEL.

Method Invocations Mule Expression Language uses standard Java method invocation. We can provide a fully qualified class name or import a class and use the unqualified name.

MEL automatically imports a number of Java classes. Eg. message.payload.getName(). If payload is a Java object—for example, representing a

person—this code invokes its getName method. The value of the expression is the value that getName returns—presumably a string representing the person’s name.

MEL Operators MEL operators follow standard Java syntax, but operands are always by value, not by

reference. For example, "A" == 'A' evaluates to true, whereas the same expression evaluates to false in Java.

Arithmetic OperatorsMule Expression Language – Arithmetic Operators

Symbol Definition Example/Value

- Minus. The value is the value of the first operand minus the value of the second.

2 - 4 -2

% Modulo. The value is the remainder after dividing the value of the first operand by the value of the second.

9 % 4 1

/ Over. The value is the value of the first operand divided by the value of the second.

2 / 4 0.5

+Plus. For numbers, the value is the sum of the values of the operands. For strings, the value is the string formed by concatenating the values of the operands.

2 + 4 6 'fu' + 'bar' The String "fubar"

* Times. The value is the product of the values of the operands.

2 * 4 8

Comparison OperatorsMule Expression Language – Comparison Operators

Symbol Definition Example/Value

== Equal. True if and only if (iff) the values of the operands are equal.

'A' == 'A' true

!= Not equal. True iff the values of the operands are unequal. 'A' != 'B' true

> Greater than. True iff the value on the left is greater than the value on the right.

7 > 5 true

< Less than. True iff the value on the left is less than the value on the right

5 < 5 false

>= Greater than or equal. True iff the value on the left is greater than or equal to the value on the right.

5 >= 7 false

<= Less than or equal. True iff the value on the left is less than or equal to the value on the right.

5 <= 5 true

contains Contains. True iff the string on the right is a substring of the string on the left.

'fubar' contains 'bar' true

is, instance of

Is an instance of. True iff the object on the left is an instance of the class on the right.

'fubar' is String true

Comparison Operators - contd

Logical Operators

Mule Expression Language – Logical Operators

Symbol Definition Example/Value

strsimDegree of similarity. The value of the expression is a number between 0 and 1 representing the degree of similarity between the two string arguments.

'foo' strsim 'foo' 1.0 ‘foobar’ strsim ‘foo’ 0.5

soundslike Sounds like. True iff the two string arguments sound alike according to a Soundex comparison.

'Robert' soundslike 'Rupert' true

Symbol Definition Example/Value

and Logical AND. True iff both operands are true. (Don’t use &&)

(a == b) and (c != d) true iff a =b and c ≠ d

|| Logical OR. True iff at least one operand is true. true ||anything Always true

or Chained OR. Scans left to right and returns the value of the first non-empty item

false or '' or ' ' or 'dog' The String "dog"

Mule Expression Language – Assignment, LiteralsMEL Assignment An assignment is a MEL expression consisting of an identifier representing a mutable object to

the left of an equal sign and a MEL expression to the right of the equal sign. Eg: message.payload = 'fu‘ sets the payload of the current message to the string "fu".

MEL determines types dynamically, so declaring the type of a variable is optional. For example if, with no prior declarations, if we write number = 1; number == '1' MEL assigns the expression the value true.

We can cast values to specific types. For example if we writenumber = (String)1; number is String MEL returns the value true for this expression.

MEL Literals Literals in MEL can be strings, numbers, Boolean values, types, and nulls. Maps, Lists and

Arrays data structures as literals as well. MEL Literals – Numeric Literals Numeric literals are integers and floating point numbers, with the same ranges of values as the

underlying Java system.MEL Literals – String Literals String literals are sequences of characters enclosed in single quotes. We cannot use double

quotes to express String literals as we can in Java, because MEL expressions appear within double quotes in configuration files.

Mule Expression Language – LiteralsMEL Literals – Boolean Literals Boolean literals are the values true and false. These are case sensitive.MEL Literals – Null Literals A null literal takes the form null or nil. These are case sensitive.MEL Literals – Type Literals Refer to any Java class by its fully qualified name or if it is one of the classes in the below

classes, by its unqualified name. References use the same dot notation as in Java, except that we must use $ rather than a dot to refer to a nested class.

MEL automatically imports the following Java classes. java.lang.* java.io.* java.net.* java.util.* java.math.BigDecimal java.math.BigInteger javax.activation.DataHandler javax.activation.MimeType java.util.regex.Pattern org.mule.api.transformer.DataType org.mule.transformer.types.DataTypeFactory

MEL Key/Value Maps, Lists, and Arrays Maps are important in Mule Expression Language because much of the context you can work

with comes in the form of maps.

Mule Expression Language – Maps, Lists and ArraysMEL Key/Value Maps, Lists, and Arrays Mule Expression Language uses a convenient syntax for maps and other data structures. It

begins with map literals, and there is also a convenient way to access items in maps. MEL provides a streamlined way to access map data. Rather than constructing a map with a new statement, and then using its put method to populate it, we can simply write the following:

[key1 : value1, key2 : value2, . . .] and use this literal form wherever we would otherwise use a map by name, including as a method argument.

similar literal forms for lists ([item1, item2, . . .]) and arrays ({item1, item2, . . .}). Arrays in Java must specify the type of their contents, but in MEL they are untyped. MEL

supplies the correct type when we use them – either by determining it at compile time or coercing the array to the correct type at run time.

Accessing Map Data MEL provides a simpler way to access map items than java.util.Map provides. For example,

Mule associates a map containing properties set by the inbound endpoint processor with each message. We can refer to this map as message.inboundProperties. To retrieve the inbound property with key name foo, write 'message.inboundProperties[foo]'. If that property can be set (never the case with inbound properties, but true of some properties in other maps), we can write message.outboundProperties[foo]=message.payload on the left side of an assignment. If we try to set a property that cannot be set,Mule indicates failure by throwing org.mvel2.PropertyAccessException.

Mule Expression Language–Control Flow, Context ObjectsControl Flow MEL provides a full range of Java control flow statements. The most useful for typical MEL

expressions are conditional operands (often called ternary statements). A conditional operand has the form condition ? true value : false value.

For example, x = (name == 'Smith' ? 'Smith' : 'Unknown') sets the variable x to the string "Smith" if the value of name is "Smith" and to the string "Unknown" if the value of name is not "Smith".MEL Context Objects and Functions Property expressions facilitate the use of properties of the Mule message and its environment

as operands in MEL expressions. They take the form contextObject.property. Context objects provide logical groupings of the properties of the message and its environment.

Functions provide ways to extract information that doesn’t already exist as a single value that can be embodied in a property.

MEL Context Objects Context objects model the message and its environment. They make MEL Mule-centric, not

just another expression language. Different context objects are Server: properties of the hardware, operating system, user, and network interface. Mule: properties of the Mule instance. App: properties of the Mule application. Message: properties of the Mule message - [DEFAULT]

Mule Expression Language – Context Object - ServerServer This object provides read-only access to the properties of the hardware, operating system, user, and

network interface listed in the table. For example, the value of 'server.userName' is a string representing the name of the user.

Name Description

fileSeparator Character that separates components of a file path ( "/" on UNIX and "\" on Windows)

host Fully qualified domain name of the serverip The IP address of the server

locale Default locale (of type java.util.Locale) of the JRE (can access server.locale.language and server.locale.country)

javaVersion JRE versionjavaVendor JRE vendor nameosName Operating system nameosArch Operating system architectureosVersion Operating system versionsystemProperties Map of Java system propertiestimeZone Default TimeZone (java.util.TimeZone) of the JREtmpDir Temporary directory for use by the JREuserName User nameuserHome User home directory

userDir User working directory.For example, the value of 'server.userName' is a string representing the name of the user.

Mule Expression Language – Context Object – Mule, AppMule This object provides read-only access to the properties of the Mule instance listed in the table. For example, the value of 'mule.version' is a string representing the Mule version.

App This object provides access to the properties of the Mule application listed in the table For example, the value of 'app.name' is a string representing the application name. For example, 'app.registry['foo']' refers to the object named foo in the Mule registry map. You

can set or retrieve its value.

Name DescriptionclusterId Cluster IDhome File system path to the home directory of the mule server installationnodeId Cluster node IDversion Mule Version

Name Descriptionencoding Application default encoding (read-only)name Application name (read-only)standalone True if Mule is running standalone (read-only)workdir Application work directory (read-only)

registryMap representing the Mule registry (read/write).For example, the value of 'app.name' is a string representing the application name.For example, 'app.registry['foo']' refers to the object named foo in the Mule registry map. You can set or retrieve its value.

Mule Expression Language – Context Object – MessageMessage This object provides access to the properties of the Mule message listed in the table.

Name Descriptionid (read-only)rootId (read-only)correlationId (read-only)correlationSequence (read-only)correlationGroupSize (read-only)replyTo (read/write)dataType (read-only)payload (read/write)inboundProperties Map (read-only)inboundAttachments Map (read-only)outboundProperties Map (read/write)outboundAttachments Map (read/write)exception (read-only)

Mule Expression Language – VariablesVariables In addition to local MEL variables, whose scope is the current message processor, MEL gives

you access to Mule flow and session variables. The variables reside in the following maps, which are available to use in MEL expressions:

flowVars – contains variables that are global to the current flow. They retain their values as control passes from one message processor to another. Thus, you can set them in one message processor and use them in another.

SessionVars – is essentially the same as flowVars, except that when one flow calls another one via a Mule endpoint they are propagated.

For example, to access the value of the foo flow variable, write flowVars['foo']. This can appear on either side of an assignment. For example, the following code gets the value of the session variable bar and uses it to set the value of the flow variable bar.

flowVars['foo'] = sessionVars['bar'] As a further shortcut, you can simply use foo as a variable name in a MEL expression. If there

is no local MEL variable called foo, the MEL processor looks for one in flowVars, then in sessionVars, before failing. For example, if the MEL expression contains foo == 'cat' and there is no local MEL variable named foo, but there is a foo key in flowVars, then the foo in the expression is equivalent to flowVars['foo'].

Note, however, that we can turn this method of resolution off by including a configuration attribute in the xml configuration file:

<configuration> <expression-language autoResolveVariables="false"></configuration>

Mule Expression Language – Data Extraction FunctionData Extraction Function The functions xpath and regex provide ways of extracting context information extract

information that doesn’t already exist as a single value that can be embodied in a property. By default they work on the payload, but we can pass them different arguments explicitly.

Thank you